Kevin is 2 times as old as Luis. Twenty years ago, Kevin was 6 times as old as Luis. How old is Luis now?
We can use the given information to write down two equations that describe the ages of Kevin and Luis. Let Kevin's current age be $k$ and Luis's current age be $l$ The information in the first sentence can be expressed in the following equation: $k = 2l$ Twenty years ago, Kevin was $k - 20$ years old, and Luis was $l - 20$ years old. The information in the second sentence can be expressed in the following equation: $k - 20 = 6(l - 20)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $l$ , it might be easiest to use our first equation for $k$ and substitute it into our second equation. Our first equation is: $k = 2l$ . Substituting this into our second equation, we get: $2l$ $-$ $20 = 6(l - 20)$ which combines the information about $l$ from both of our original equations. Simplifying the right side of this equation, we get: $2 l - 20 = 6 l - 120$ Solving for $l$ , we get: $4 l = 100.$ $l = 25$.